edit: I found another workaround that appears a little cleaner to me:
the XEVIE extension is required for accessibility features. Reemerging gdm without the accessibility use flag did the trick for me.

well it may have been required for accessibility features eons ago, but it hasn't existed in Xorg since end of 2008. Unfortunately, the current stable gdm in Portage is quite old, gnome-base/gdm-2.32.1 is hardmasked due toy multiple regressions, and anything newer pulls in the rest of GNOME 3.